A catalyst suited for reducing NOx in an exhaust gas by ammonia in the presence of excess oxygen is arranged in an engine exhaust passage. An aqueous urea solution is fed from a flow addition valve onto a catalyst, part of the urea fed to the catalyst is stored in the catalyst, and the ammonia generated from the urea stored in the catalyst is used to reduce the NOx in the exhaust gas. At the time of engine startup, at the time of cold operation, and at the time of increase of the HC amount, it is determined that the amount of formaldehyde in the exhaust gas flowing into the catalyst exceeds the allowable amount, and, at this time, the feed of urea to the catalyst is prohibited.